Eukaryotic cells have developed an elegant system called RNA silencing for getting rid of foreign RNAs whether they be of viral, retrotransposon, or transgene origin. In his Perspective, [Baulcombe][1] examines new findings ([ Wu-Scharf et al .][2]) showing that in a green alga the gene responsible for RNA silencing encodes an RNA helicase (related to proteins in worms and other organisms) that is required for regulation of gene expression at the RNA level. [1]: http://www.sciencemag.org/cgi/content/full/290/5494/1108 [2]: http://www.sciencemag.org/cgi/content/short/290/5494/1159